This invention relates generally to surgical joint prosthesis and especially to implant devices that are intended for substantially total replacement of the surfaces of a diseased or traumatized human knee.
Much medical interest in total joint replacement has been generated in the course of the past decade, originally stimulated to a large degree by the successes of John Charnley with total hip joint replacement. The concept of total joint replacement has been extended from hips to encompass knees, shoulders, ankles and elbows. Moreover, total joint repair has been increasingly applied to younger patients. Not unexpectedly, the number of prosthesis arrangements has proliferated apace with these conceptual advances and the United States patent literature is replete with examples of knee replacement prosthesis, including the following: Moritz U.S. Pat. No. 3,694,821; Herbert et al U.S. Pat. No. 3,795,922; and Kaufer et al U.S. Pat. No. 3,868,720.
The human knee is generally recognized as being an inherently unstable joint which is normally capable, nevertheless, of accommodating complex motions and successfully accepting substantial physical stress loads. Despite the rather appreciable efforts made in the past to develop an artificial total knee replacement prosthesis and despite the variety of approaches that have been pursued, results have not heretofore been entirely satisfactory. Prior efforts have left unsolved the problem of preserving the strength and stability required when the joint is extended without sacrificing the complex motions found when the natural knee joint is in flexion. This is particularly true when the cruciate ligaments have been destroyed due to disease or trauma.
Knee prosthesis are divided generally into two types or groups, the first of which is the non-constrained or unlinked device. Here, the damaged load bearing surfaces of the knee are replaced with plastic or metal caps or molds and the stability of the joint is provided by the surrounding, intact ligaments and muscles. The non-constrained devices are often effective because of their wide load bearing surfaces and approximation of natural joint movement. In addition, as only the articulating surfaces of the knee joint are replaced, the bone removal upon installation is minimal. Thus, the inherent strength of the surrounding bone supports the prosthesis and the availability of future surgical alternatives is retained. However, if the remaining ligaments are insufficient to insure joint stability or if the articular surfaces are severly impaired, a prosthesis with a mechanical linkage between the femoral and tibial components has heretofore been required. These latter devices, termed "constrained", necessitate substantial bone removal with attendant difficulties upon any subsequent surgical revision.
Examples of the constrained or linked design include the Walldius prosthesis which employs a simple fixed horizontal hinge to afford joint stability in the absence of supporting ligaments. However, since, during flexion of the natural knee joint, the center of rotation moves in a posterior spiral curve, the single axis hinge in the Walldius-type knee cannot approximate the complex polycentric rotation of a normal knee. In addition, as a fixed hinge is rigid in all positions, The Walldius design cannot allow the axial and lateral freedom of movement found in the natural knee.
The patent to Moritz U.S. Pat. No. 3,694,821 discloses a multi-chambered ball-and-socket joint for more closely approximating these polycentric tri-axial movements. However, ball-and-socket joints designed small enough to be placed within the human knee have proved oftentimes to be structurally insufficient to sustain the dynamic loads imposed upon the knee joint during normal activities. Also, due to the spherical shape of the articulating portion, when such a joint is implanted, considerable bone must be removed, especially from the distal femur; and this results in a loss of structural support for the prosthesis and substantially limits future surgical alternatives. In addition, while the Moritz ball-and-socket device permits triaxial movement when the joint is in flexion, it fails to limit such movements upon extension to provide the rigid lateral stability of the extended natural knee.
The device of the patent to Herbert et al U.S. Pat. No. 3,795,922 attempts to provide extensile stability in a ball-and-socket prosthesis by the use of engaging locking members disposed between the femoral and tibial components. However, in addition to suffering from the above-mentioned structural deficiencies inherent in all ball-and-socket prostheses, the Herbert device requires substantially more bone removal to accommodate the additional locking members.
The patent to Kaufer et al U.S. Pat. No. 3,868,730 teaches the use of primary load bearing surfaces arranged on either side of the ball-and-socket linkage which are similar in shape to those in the natural knee joint. However, the Kaufer prosthesis posesses the disadvantage inherent in all of the linked or constrained knee prostheses heretofore employed, i.e. the centers of rotation of the prosthesis are substantially above the tibial plateau and the ball and the socket must be implanted well into the femur to approximate natural movement. Also, as the Kaufer knee provides a plastic lining or insert between the ball and the socket, the overall size of the femoral member is accordingly increased and greater amounts of supporting bone must be removed when the knee is implanted. In addition, the femoral socket member is generally supported by an elongate fixation stem which is driven superiorly into the cortical bone of the femur.
Thus, the linked prosthetic knee requires a substantial sacrifice of supporting bone structure. After such a device is installed, future surgical alternatives become limited and fusion of the knee or arthrodesis becomes virtually impossible.
It is therefore a general object of the present invention to provide a new and improved knee prosthesis.
Another object of the present invention is to provide a surface replacement knee prosthesis which mechanically limits dislocation of the load bearing surfaces.
Another object of the present invention is to provide a total knee prosthesis which affords axial and lateral rotation in flexion while substantially limiting such rotations upon extension.
Still another object of the present invention is to provide a semi-restraining knee prothesis which requires minimal bone removal and resectioning upon installation.